This relates generally to imaging devices, and more particularly, to imaging devices with vertically stacked photodiodes controlled using vertical transfer gates.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns.
Conventional image sensors include photodiodes formed using dopant implantation. In some arrangements, stacked photodiodes are formed by implanting n-type regions at different depths in a p-type substrate. A blue photodiode is formed at a first depth in the substrate; a green photodiode is formed at a second depth in the substrate that is greater than the first depth; and a red photodiode is formed at a third depth in the substrate that is greater than the second depth. Moreover, the blue photodiode is formed within a first region on the substrate; the green photodiode is formed within a second region on the substrate that is non-overlapping with the first region; and the red photodiode is formed within a third region on the substrate is non-overlapping with the first and second regions. Each of these regions is connected to a top node through respective large photodiode trunk structures.
While vertically stacked photodiodes formed in this way might be capable of resolving red, green, and blue colors without color filter arrays, these photodiodes exhibit undesired optical and electrical crosstalk due to the large trunk structures. It is also difficult to precisely control the depth and dopant profiles of the different stacked photodiodes and their trunk structures. Furthermore, this configuration is not scalable to smaller dimensions since many trunk structures and gates are needed.